Typical aircraft turbofan jet engines include an engine core, a nacelle that surrounds the engine core, and a fan that draws in a flow of air that is split into bypass airflow and engine core airflow. The nacelle provides a bypass duct that surrounds the engine core. The bypass airflow is transported through the bypass duct. The nacelle is configured to promote laminar flow of air through the bypass duct. The engine core includes a multi-stage compressor to compress the engine core airflow, a combustor to add thermal energy to the compressed engine core airflow, and a turbine section downstream of the combustor to produce mechanical power from the engine core airflow. The typical turbine section has two and sometimes three turbine stages. The turbine stages are used to drive the compressor and the fan. After exiting from the turbine section, the engine core airflow exits through an exhaust nozzle at the aft end of the engine.
In a turbofan engine, the fan typically produces a majority of the thrust produced by the engine. The bypass airflow can be used to produce reverse thrust typically used during landing. Thrust reversers mounted in the nacelle selectively reverse the direction of the bypass airflow to generate reverse thrust. During normal engine operation, the bypass airflow may or may not be mixed with the exhausted engine core airflow prior to exiting the engine assembly.
Several turbofan engine parameters have a significant impact upon engine performance. Bypass ratio (BPR) is the ratio of the bypass airflow rate to the engine core airflow rate. A high BPR engine (e.g., BPR of 5 or more) typically has better specific fuel consumption (SFC) and is typically quieter than a low BPR engine of equal thrust. In general, a higher BPR results in lower average exhaust velocities and less jet noise at a specific thrust. A turbofan engine's performance is also affected by the engine's fan pressure ratio (FPR). FPR is the ratio of the air pressure at the engine's fan nozzle exit to the pressure of the air entering the fan. A lower FPR results in lower exhaust velocity and higher propulsive efficiency. Reducing an engine's FPR can reach a practical limit, however, as a low FPR may not generate sufficient thrust and may cause engine fan stall, blade flutter, and/or compressor surge under certain operating conditions.
One approach for optimizing the performance of an engine over various flight conditions involves varying the fan nozzle exit area. By selectively varying the fan nozzle's exit area, an engine's bypass flow characteristics can be adjusted to better match a particular flight condition, for example, by optimizing the FPR relative to the particular thrust level being employed. For example, a variable area fan nozzle (VAFN) that forms a rear outer portion of the bypass duct can be moved aft so as to open an additional bypass flow exit forward of the VAFN. The VAFN can be selectively positioned anywhere between a stowed position in which no additional bypass exit is formed and a fully deployed position in which the additional bypass exit is open to a maximum extent.
Integrating a VAFN into an engine nacelle, however, presents challenges that arise from conflicting goals. In the stowed position, it is preferable that the VAFN interfaces with the rest of the nacelle such that the additional bypass exit is closed and sealed without inducing high stowing related loads in actuators used to position the VAFN. Accordingly, to meet stowed position sealing goals, it is desirable to have interfacing components with low stiffness. In the deployed position, however, it is preferable that the resulting additional bypass exit has desirable aerodynamic characteristics, such as low drag. Accordingly, to meet deployed position aerodynamic goals, it is desirable that the foregoing interfacing components be sufficiently stiff to avoid undesirable deflections, which can cause aerodynamic drag.
Accordingly, improved interfacing components for a VAFN are desired, such as a primary seal assembly having good stowed and deployed position characteristics.